1. Field of the Invention
The present invention relates to an image forming method that forms a polymer image by the action of light, and also to an image forming medium.
2. Related Background Art
Energies used to form or record an image include light, sound, electricity, magnetism, heat, radiations such as electron rays and X-rays, and chemical energy, among which, in particular, widely used are light, electricity, heat energy, or a combination of any of these.
For example, the image forming method that employs the combination of light energy with chemical energy includes a silver salt photographic process and a method in which a diazo copying paper is used. The method that employs the combination of light energy with electric energy includes an electrophotographic system. Also, the method that utilizes heat energy includes a method in which a thermal recording paper or transfer recording paper is used. On the other hand, known as the method that utilizes electric energy is a method in which an electrostatic recording paper, electrothermal recording paper, or electrosensitive recording paper is used.
Of the above image forming methods, the silver salt photographic process can obtain an image having a high resolution. The silver salt photographic process, however, requires the developing and fixing that uses complicated liquid compositions, or the drying of an image (or a print).
Now, development is energetically made on image forming methods that can form an image through a simple processing.
For example, U.S. Pat. No. 4,629,676 teaches a method in which polymerization reaction under dry (thermal) conditions is caused by the photosensitive reaction of silver halide that acts as a trigger, to form an image comprising a polymer.
This method has the advantage that any complicated wet processing is not required, but has had the disadvantage that the polymer formation rate (i.e., polymerization rate of a polymeric compound) is so low that it takes a long time to form the polymer image. Incidentally, this disadvantage arises presumably because of a reaction intermediate (which functions as a polymerization initiator) formed in the course of heating, by the reaction between silver produced from silver halide by imagewise exposure and a reducing agent, which intermediate is so stable and has so low activity as the polymerization initiator that the polymerization reaction can not proceed so rapidly.
On the other hand, to cope with this problem to accelerate the polymerization, Japanese Unexamined Patent Publication No. 70836/1987 discloses a method in which a thermal polymerization initiator is used.
This method comprises forming a latent image comprising silver metal produced from silver halide by imagewise exposure, converting, under heating, a reducing agent into an oxidized product having a polymerization inhibitory power different from that of said reducing agent by utilizing a catalytic action of the above silver metal, thereby producing a difference in the polymerization inhibitory power between the reducing agent and the resulting oxidized product and also causing a thermal polymerization reaction utilizing the thermal polymerization initiator, thus forming a polymer image corresponding with the difference in the polymerization inhibitory power.
This method, however, has been involved in the disadvantage that a good contrast can be made with difficulty in the polymer image.
This disadvantage arises presumably because the oxidation-reduction reaction taking place in a latent image portion to form the oxidized product and the polymerization reaction to form the polymer image are allowed to take place in the same heating step, so that these reactions may proceed in a competitive fashion and thus the respective reactions may not proceed in a good efficiency.
Also, the image formation according to this method is very unstable in that, for example, the areas on which the polymer is formed may turn into exposed areas or unexposed areas only because of a slight change in the amount of the reducing agent.
In addition, U.S. Pat. No. 4,649,098 discloses a method in which a reducing agent having a polymerization inhibitory power is brought into an oxidized product by imagewise consumption (at imagewise exposed areas) in the course of the developing of silver halide, and, after imagewise inhibition (at imagewise unexposed areas) of polymerization reaction by the action of the residual reducing agent, light energy is uniformly applied (whole areal exposure) from the outside to cause photopolymerization at the area at which the reducing agent has been consumed (imagewise exposed area), thus forming a polymer image.
The above method has the advantages that it can achieve a high sensitivity in the writing of a latent image since the silver halide is used, and the steps from the writing for the formation of an image up to the whole areal exposure can be separated in a good efficiency. It, however, is difficult to obtain a polymer image having a sufficient contrast. This is caused for the following reason.
The reducing agent used in the above method is in itself a reducing agent that acts as a polymerization inhibitor and turns not to act as the polymerization inhibitor after the reduction of silver halide. Hence the reducing agent at the imagewise exposed area must be sufficiently converted into the oxidized product before the polymerization can be sufficiently achieved. However, the application of heat energy in an sufficient amount in carrying out the development, with the intention to sufficiently convert the reducing agent at the imagewise exposed area into the oxidized product may cause an unauthorized oxidation-reduction reaction at the imagewise unexposed areas. On the other hand, the application of heat energy in a reduced amount in carrying out the development, with the intention to prevent the oxidation-reduction reaction from taking place at the imagewise unexposed area, may conversely make the conversion into the oxidized product at the imagewise exposed areas not to sufficiently proceed. Since in this instance the imagewise exposed area of an oxidation-reduction image is polymerized with difficulty, the light energy in carrying out the whole areal exposure must be applied in an increased amount. This may cause unnecessary polymerization at the unexposed areas with increase in the amount of the light energy. Further, since the silver metal is deposited at the area to be polymerized, the light energy of the whole areal exposure is absorbed so that the imagewise exposed area and unexposed area become different in the point of the irradiation energy. As a result, the polymerization at the unexposed area is more apt to proceed, eventually making it impossible to obtain the polymer image with a sufficient contrast.
The polymer image to be formed according to the methods as described above is an image comprising a polymerized area and an unpolymerized area. Aiming at making this polymer image visible and further forming it into a color image, U.S. Pat. No. 4,649,098 and so forth disclose various methods that utilize the difference in properties and so forth between the polymerized area and unpolymerized area. For example, proposed are a method in which a treatment is made using a liquid that does not dissolve the polymerized area and dissolves the layer of the unpolymerized area, to dissolve out and remove the unpolymerized area (i.e., etching); a method in which, utilizing the difference in adhesion between the polymerized area and unpolymerized area, a sheet such as plastic film is adhered and thereafter peeled to separate the polymerized area and unpolymerized area under dry conditions (i.e., peeling-apart); in the case when the polymer image is formed into a color image, a method in which a photopolymerizable layer is previously colored with use of a pigment or dye, which is then subjected to dissolving-out (i.e., the above etching) or peeling (i.e., the above peeling-apart) to form the color image, or a method in which, utilizing the adhesion at the unpolymerized area, a coloring powder is applied to make selective coloring (i.e., torning or inking), or, utilizing the difference in liquid-permeability between the polymerized area and unpolymerized area, the unpolymerized area is selectively dyed by treating it with a dye solution.
However, no polymer image having a sufficient contrast can be obtained in the conventional polymer image forming methods as discussed above, and hence, even with employment of any of the above methods for making the image visible or forming it into a color image, the visible image and color image which are obtained from such a polymer image can not have any sufficient contrast, and particularly it has been difficult to obtain highly detailed visible image and color image.
Japanese Unexamined Patent Publication No. 55-50246 also discloses a photosensitive lithographic material that has a photopolymerizable composition layer, a transparent intermediate layer and a heat development type photosensitive composition layer, and is capable of forming a polymer image in the photopolymerizable composition layer with utilization of the light-absorption of the metallic silver which is produced at the exposed area of the heat development type photosensitive composition layer as a result of imagewise exposure and heating (a plate is finally obtained by peeling-apart).
The metallic silver, however, has so flat light-absorption characteristics that it is hard to say that the ability to absorb a particular wavelength is satisfactory. Hence, it is necessary for achieving sufficient absorption of light to make the photosensitive layer thick. This results in a lowering of the resolution of the polymer image. When a method is employed in which the quantity of the organic silver salt per unit area is increased to achieve sufficient absorption of light, there also has been the problem that the storage stability of image forming mediums is lowered.